She stepped into the Lower Torso Assembly. The unit was heavy, the layered material awkward and stiff, and it seemed to wriggle away from her legs as Stone tried to pull it up for her. She found she was tiring rapidly, already.

Next she fitted a tube over her catheter attachment. It would connect with a bag large enough to store a couple of pints of urine. There was nothing to collect shit, though; she was wearing a kind of diaper — an absorbent undergarment — that would soak up “any bowel movement that cannot be deferred during EVA,” in the language of the training manuals.

York planned to defer.

Then it was time for the Hard Upper Torso. Her HUT was suspended from the wall of the airlock, like the top half of a suit of armor, with a built-in life support backpack.

She crouched down underneath the HUT and lifted her arms. She wriggled upward, squirming into the HUT. In the darkness of the shell there was a smell of plastic and metal and lint, of newness.

She got her arms into the sleeves and pushed her hands through; the cooling garment’s loops tugged at the soft flesh around her thumbs. Her shoulders bent backwards, painfully. Nothing about this process was easy. Still, these suits were a hell of a lot simpler than the old Moon suits; the Apollo crew members had had to assemble their suits on the lunar surface, connecting up the tubes which would carry water and oxygen from their backpacks.

Her head emerged through the helmet ring. Stone was grinning at her. “Welcome back.” He pulled her HUT down, jamming it so that it rubbed against her shoulders, and guided the metal waist rings of the two halves of the suit to mate and click together.

Then she helped Stone don his suit.

York and Stone had already been inside the cramped airlock for two hours. Challenger’s atmosphere was pressurized to 70 percent of Earth’s sea level, with a mix of nitrogen and oxygen, but to stay flexible their suits would contain oxygen only, at just a quarter of sea-level pressure. So York and Stone had had to prebreathe pure oxygen to purge the nitrogen from their blood.

It was a tedious ritual. And EVAs on Mars could last only three or four hours, at most. Apollo backpacks had been capable of supporting seven hours of surface working. But Mars’s gravity was twice as strong as the Moon’s, and Mars suits had to be proportionately lighter, and could therefore only sustain much briefer EVAs. There would also have to be a long tidy-up period after each EVA: the suits would have to be vacuumed clean of Mars dust, which was highly oxidizing and would play hell with their lungs if they let it into Challenger.

The brief EVAs, with the surrounding preparation and cleanups and anticontamination swabbing, were going to occupy most of each exhausting, frustrating day on Mars.

York fixed on her Snoopy flight helmet, and over the top of that Stone lifted her hard helmet, with its visor, and twisted it into place against the seal at her neck.

The last pieces were her gloves; these were close-fitting and snapped onto rings at her wrists.

Stone flicked a switch on her chest panel, and she heard the soft, familiar hum of pumps and fans in the backpack, the whoosh of oxygen across her face. He rapped sharply on the top of the helmet and held up a gloved thumb before her clear faceplate. She nodded out at him and smiled.

She held up her arm; there was a reflector plate stitched into her cuff, allowing her to see the panel on the front of her chest which gave her a readout of oxygen, carbon dioxide and pressure levels, and various malfunction warnings. She could see her oxygen pressure level stabilizing.

Stone tested out the radio link. “Hi, Natalie. Able Baker Charlie…” His voice sounded soft and tinny, echoed by muffled sound carried through the thick glass of her faceplate.

She checked the small plastic tubes protruding from her helmet’s inner surface; she sipped out little slugs of water and orange juice. The OJ was okay, but the water was too warm. It didn’t matter. She pushed her suit’s internal pressure up to maximum, briefly, to test for leaks. She fixed her little spiral-bound EVA checklist to her cuff.

When they were through with the suit checkout they studied each other. Stone’s suit was gleaming white, with bright blue Mars overboots, and the Stars and Stripes proudly emblazoned on his sleeves.

Stone asked: “Are we done?”

She was sealed off from Challenger: locked inside her own, self-contained, miniature spacecraft. She took a deep breath of cool, blue oxygen. “Yes. Let’s get on with it.”

“Roger.” He looked away from her to talk to Gershon, who was up in the ascent stage. “Ralph, we’re waiting for a Go for depress on time.”

“Rager, Phil; you have a go for depress.” Gershon would monitor this first EVA from the ascent-stage cabin.

Stone closed a switch on the wall; York heard sound leak out of the air, and the internal noise of her own breathing seemed to grow louder, more ragged, to compensate.

“Roger,” Stone said. “Everything is go here. We’re just waiting for the cabin to bleed enough pressure to open the hatch.”

The gauge, York saw, showed the pressure already down to two-tenths of a pound.

Gershon said, “I’m reading a real low static pressure on your lock. Do you think you can open the hatch yet?”

Stone said, “I’ll try.”

The exit from the airlock was a small hatch, close to the floor. The handle was a simple lever. Stone bent down, twisted the handle, tugged. York could see the thin metal of the hatch bow inward. The hatch stayed shut.

“Damn it.”

“Let me try.” She crouched down and picked at the corner of the hatch, where it protruded from the wall. Her gloves, of metal mesh and rubber, were clumsy; her hands felt huge and insensitive. But she managed to get a little flap of the hatch peeled back.

Through the sliver she’d opened up between the hatch and its frame, she could see ocher light.

“I think I broke the seal.”

Stone pulled at the handle, and this time the hatch opened easily.

York saw a little flurry of snow as the last of their air escaped into the Martian atmosphere.

They both had to back away to let the hatch swing back.

Then York could see the porch, the platform fixed to the top of Challenger’s squat landing leg, onto which Stone would back out in a moment. The porch was coated in brown grit, thrown up by the landing. And beyond the porch, she could see the surface of Mars: it looked like sand, and it was streaked with radial lines pointing away from Challenger, showing the effects of their descent engine’s final blast.

It was just a scrap of landscape; on Earth it would look so commonplace she wouldn’t even perceive it. But it was Mangala Vallis: and there were only a few feet of thin Martian air separating her from the surface she’d been studying all her adult life.

“Natalie,” Stone said.

She turned; in contrast to the brown of Mars, in the mundane kitchen-light of the airlock, his suit seemed to glow white.

“There’s something we forgot,” Stone said. “From the checklist. We didn’t fix these.” Stone had taken his red EV1 bands from a suit pocket. Stone, as the leader of the first EVA, was in charge of the operation; York was, officially, his backup, and Stone would wear the red bands around his arms and legs for identification by the TV cameras.

But he was holding the bands out to her.

“I don’t understand.”

He was smiling again. “I think you do. Put on the bands.”

She held out her hand, and he dropped the bands into her palm. Through her clumsy gloves she couldn’t feel the bands’ weight.

“You’ve got to be kidding.”

He said testily, “Look, I’m not asking you to land the goddamn MEM. You’ve done this in the contingency sims. All you have to do, on this first EVA, is to walk around and scratch a few rocks, and talk to the folks at home about it.”